If you check your log you'll see that a mob "partially resists" (or completely, but not often) your spell. This will prevent the combo from triggering. Magic Accuracy affects this. Since the combo damage from Thundara is so potent, THMs and BLMs will typically shoot for 470 M.ACC or higher so that the combo works most or all of the time. This number is before party bonuses, but can include the effects of food or BRD's Minuet of Rigor, which frees up more gear slots for additional potency and INT.

M.ACC also affects how easy it is to land Sleep and Sleepga.